Time Domain Terahertz Spectroscopy (TDTS) of gases is known. Time Domain Terahertz Spectroscopy utilizes substantially the same procedure used in the more traditional Time Domain spectroscopy (TDS), which is performed at optical frequencies.
In either instance (TDTS or TDS), the frequency of radiation from a narrowband source is swept through a given frequency range. In general practice the radiation (infrared, visible, ultraviolet, etc) is transmitted through a sample of the material to be analyzed. The sample is typically contained within a sample cell.
An absorption spectrum is measured using radiation that has passed through the sample. A photodetector or another type of sensor that is sensitive to frequencies within the frequency range being scanned is used to measure the absorption spectra.
High resolution spectroscopy requires the use of a tunable radiation source. The radiation source must have a very narrow line width in order to provide the desired resolution. The maximum scan rate that may be used depends upon characteristics (such as sensor bandwidth) of the detector used. However, the resolution of minute variations of absorption as a function of frequency may require that the scan rate be significantly reduced.
When a large number of different species of gases coexist in the sample, the required spectral resolution of the measuring instrument is greatly increased. Better spectral resolution is necessary in order to identify each species and relative strengths (partial pressures) thereof. This is true for the spectroscopic measurement of gases in the terahertz regime, as well as when using more conventional frequencies.
Key characteristic lines of the absorption spectra of gases with complex (large) molecules tend to lie in the far infrared (sub-millimeter wave) portion of the electromagnetic spectrum. Many such lines may be crowded into this portion of the electromagnet spectrum, making the resolution of individual lines difficult.
One problem associated with contemporary attempts at implementing Time Domain Terahertz Spectroscopy is that associated with obtaining high quality terahertz radiation sources. These sources need to have narrow line-widths, adequate power, and be rapidly tunable.
Further, sensitive and high speed terahertz detectors are difficult to obtain. Restrictions on weight, volume, and cost, as well as the desire for extremely high sensitivity (parts per trillion), increase the difficulty of obtaining suitable detectors. These problems are greatly exacerbated when there is a need to handle several hundred gas species that co-exist simultaneously in a single sample.
As a result, there is a need for a terahertz spectroscopy system suitable for analyzing samples containing many gas species. It would be beneficial if the system could use terahertz radiation sources and detectors having reduced resolution requirements. Further, the terahertz spectroscopy system needs to have desirable weight, volume, cost, and sensitivity.